Hollow parts for auto body construction, such as frame members or reinforcement beams, may ideally require a varying cross-sectional shape and/or perimeter along their length. Conventional hollow parts having varying cross-section may, for example, be stamped from two pieces of sheet metal, each piece forming two longitudinal halves of the completed tube. The two pieces are then welded together with two welded seams, each weld running the length of the part. This requires a relatively large amount of labor and welding to produce the finished hollow member, thus resulting in large processing expense.
One method for producing hollow parts with varying cross section is hydroforming. The process of hydroforming metal structural components is well known. See, for example, U.S. Pat. Nos. 4,567,743, 5,070,717, 5,107,693, 5,233,854, 5,239,852, 5,333,775, and 5,339,667, the disclosures of which are hereby incorporated by reference. In a conventional hydroforming process, a tubular metal blank member is placed into a die cavity of a hydroforming die. Opposite ends of the tube are sealed, and fluid is injected under pressure internally to the tubular blank so as to expand the blank outwardly into conformance with the interior surfaces defining the die cavity. In more recent improvements to the conventional hydroforming process, opposite ends of the tubular blank are forced longitudinally toward one another during outward expansion of the tube so as to replenish the wall thickness of the metal as it is expanded outwardly. An exemplary process for replenishing material by longitudinally compressing the blank is disclosed in U.S. Pat. Nos. 5,718,048, 5,855,394, 5,899,498, and commonly-assigned 5,979,201 and 6,014,879.
An advantage to hydroforming hollow parts is that high-strength parts having irregular cross-sectional configurations can be made easily and cost-effectively, in a manner which would be extremely difficult if not impossible to accomplish using stamping or roll-forming techniques.
For some applications where a hollow part requires extreme variations in cross-section, hydroforming becomes somewhat problematic in conventional hydroforming, the cross-section diameter of the uniform cross-sectioned blank (typically cylindrical in shape) is typically chosen to be somewhat less than the smallest dimension of the part to be formed. The blank is then expanded as determined by the size of the die cavity. Where portions of the tube blank are to be expanded to very large extents (e.g., greater than 30%), the wall thickness of the tube at such locations may become overly thin to the detriment of the part.
For certain applications wherein extended portions of the part can be provided with a generally constant cross-sectional shape (e.g., as would be produced by extrusion) there is no need to subject the entire part to a hydroforming process. In addition, it may be desirable to provide a hollow part that incorporates two or more uniformed cross section tubular members (e.g., formed by extrusion or roll forming), but of different cross-sectional shapes and/or dimensions from one another. To provide such a part is problematic, however, because of the need to connect tubes having dissimilar shapes and/or dimensions.
It is therefore an object of the present invention to overcome the difficulties noted above in a novel, cost-effective manner.